Transversely-excited film bulk acoustic resonator with lateral etch stop

ABSTRACT

Acoustic resonator devices and methods are disclosed. An acoustic resonator device includes a substrate having a front surface and a cavity, a perimeter of the cavity defined by a lateral etch-stop comprising etch-stop material. A back surface of a single-crystal piezoelectric plate is attached to the front surface of the substrate except for a portion of the piezoelectric plate that forms a diaphragm that spans the cavity. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm.

RELATED APPLICATION INFORMATION

This patent claims priority from the following provisional patentapplications: application 62/867,685, filed Jun. 27, 2019, entitled XBARRESONATOR FABRICATION USING FRONT-SIDE CAVITY ETCH WITH LATERAL ETCHSTOP; and application 62/904,407, filed Sep. 23, 2019, entitled XBARFABRICATION PROCESS. This patent is related to application Ser. No.16/230,443, filed Dec. 21, 2018, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, now U.S. Pat. No. 10,491,192.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. Some of these bands are not presently used. Futureproposals for wireless communications include millimeter wavecommunication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3A is an alternative schematic cross-sectional view of an XBAR.

FIG. 3B is another alternative schematic cross-sectional view of anXBAR.

FIG. 3C is another alternative schematic cross-sectional view of anXBAR.

FIG. 4 is a flow chart of a conventional process for fabricating anXBAR.

FIG. 5A, FIG. 5B, and FIG. 5C (collectively “FIG. 5”) are a flow chartof a process for fabricating an XBAR using a lateral etch stop.

FIG. 6A, FIG. 6B, and FIG. 6C (collectively “FIG. 6”) are a flow chartof another process for fabricating an XBAR using a lateral etch stop.

FIG. 7A and FIG. 7B (collectively “FIG. 7”) are a flow chart of anotherprocess for fabricating an XBAR using a lateral etch stop.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100 as described in U.S. Pat. No. 10,491,192. XBARresonators such as the resonator 100 may be used in a variety of RFfilters including band-reject filters, band-pass filters, duplexers, andmultiplexers. XBARs are particularly suited for use in filters forcommunications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. XBARs may be fabricatedon piezoelectric plates with various crystallographic orientationsincluding Z-cut, rotated Z-cut and rotated Y-cut.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers.

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 (as shown subsequently in FIG. 3A, FIG. 3B and FIG. 3C). The cavity140 may be formed, for example, by selective etching of the substrate120 before or after the piezoelectric plate 110 and the substrate 120are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the portion 115 of thepiezoelectric plate that spans, or is suspended over, the cavity 140. Asshown in FIG. 1, the cavity 140 has a rectangular shape with an extentgreater than the aperture AP and length L of the IDT 130. A cavity of anXBAR may have a different shape, such as a regular or irregular polygon.The cavity of an XBAR may more or fewer than four sides, which may bestraight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds, possiblythousands, of parallel fingers in the IDT 110. Similarly, the thicknessof the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g.bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 is formed between the IDT fingers 238. Although not shown inFIG. 2, the front side dielectric layer 214 may also be deposited overthe IDT fingers 238. A back-side dielectric layer 216 may optionally beformed on the back side of the piezoelectric plate 110. The back-sidedielectric layer 216 has a thickness tbd. The front-side and back-sidedielectric layers 214, 216 may be a non-piezoelectric dielectricmaterial, such as silicon oxide or silicon nitride. tfd and tbd may be,for example, 0 to 500 nm. tfd and tbd are typically less than thethickness ts of the piezoelectric plate. tfd and tbd are not necessarilyequal, and the front-side and back-side dielectric layers 214, 216 arenot necessarily the same material. Either or both of the front-side andback-side dielectric layers 214, 216 may be formed of multiple layers oftwo or more materials.

The IDT fingers 238 may be aluminum, a substantially aluminum alloys,copper, a substantially copper alloys, tungsten, molybdenum, beryllium,gold, or some other conductive material. Thin (relative to the totalthickness of the conductors) layers of other metals, such as chromium ortitanium, may be formed under and/or over the fingers to improveadhesion between the fingers and the piezoelectric plate 110 and/or topassivate or encapsulate the fingers. The busbars (132, 134 in FIG. 1)of the IDT may be made of the same or different materials as thefingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e. the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness is of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3A, FIG. 3B, and FIG. 3C show alternative schematic cross-sectionalviews along the section plane A-A defined in FIG. 1. In FIG. 3A, apiezoelectric plate 310 is attached to a substrate 320. A portion of thepiezoelectric plate 310 forms a diaphragm 315 spanning a cavity 340 inthe substrate. The cavity 340 does not fully penetrate the substrate320. Fingers, such as finger 336, of an IDT are disposed on thediaphragm 315. The cavity 340 may be formed by etching the substrate 320with a selective etchant that reaches the substrate through one or moreopenings 342 provided in the piezoelectric plate 310.

FIG. 3A illustrates problems that occur when the cavity is formed byetching the substrate 320 with a selective etchant that reaches thesubstrate through the openings 342 in the piezoelectric plate 310. Theprocess used to form the cavity 340 will introduce a liquid or gaseousetchant via the holes 342. This etchant will etch isotropically, causingthe cavity to expand in all directions from the holes 342. The resultingcavity will effectively increase the area of the diaphragm 315 beyondthe area occupied by the IDT. Resistive and acoustic losses in the IDTgenerate heat in the diaphragm. Enlarging the diaphragm area beyond thearea of the IDT increases the difficulty of removing the heat from thediaphragm. Further, the bottom of the cavity 340 will not be parallel tothe piezoelectric plate 310. The deepest portions of the cavity will beproximate the holes 342 and the minimum depth of the cavity will occurnear the center of the diaphragm 315.

The shape of the cavity in FIG. 3A is an artistic rendition of thepotential problems with etching the cavity 340 with an etchantintroduced through holes in the piezoelectric plate 310. The illustratedshape of the cavity is not based on simulation or measurement of anactual etching process.

In FIG. 3B, a piezoelectric plate 310 is attached to a substrate 320. Aportion of the piezoelectric plate 310 forms a diaphragm 315 spanning acavity 340 in the substrate. The cavity 340 does not fully penetrate thesubstrate 320. Fingers, such as finger 336, of an IDT are disposed onthe diaphragm 315. The cavity 340 has been formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings 342 in the piezoelectric plate 310. Thelateral extent of the cavity 340 is defined by a lateral etch-stop 350,where “lateral” is defined as any direction parallel the surfaces of thepiezoelectric plate 110. While the lateral etch-stop 350 is shown incross-section in FIG. 3B, it should be understood that the lateraletch-stop 350 is a continuous closed band of etch-stop material thatsurrounds the cavity 340 such that the lateral etch-stop 350 defines thesize of the cavity 340 in the direction normal to the plane of thedrawing. In this patent, the term “lateral” encompasses any directionparallel to the surfaces of the substrate and piezoelectric plate.

The lateral etch-stop 350 is formed from an etch-stop material that issubstantially impervious to the process and etchant used to form thecavity 340. An etch-stop material is considered “substantiallyimpervious” if the lateral etch-stop fulfills its function ofcontrolling the lateral growth of the cavity. The etch-stop material maynot be etched by the process used to form the cavity. or may be etchedsufficiently slowly that the lateral etch-stop constrains the lateralextent of the cavity 340. When the substrate 320 is silicon, the etchantmay be, for example, XeF₂ and the etch-stop material may be, forexample, silicon oxide (SiO_(x)), silicon nitride, silicon oxynitride, ametal oxide, a metal nitride, a glass, a ceramic, or a polymer material.Different etch-stop materials and different etchants may be used withdifferent substrate materials. In all cases, the etch-stop material isdifferent from the substrate material.

The lateral etch-stop 350 constrains the growth of the cavity 340laterally but not vertically. The bottom of the cavity 340 will not beparallel to the piezoelectric plate 310. The deepest portions of thecavity will be proximate the holes 342 and the minimum depth of thecavity will occur near the center of the diaphragm 315.

In FIG. 3C, a piezoelectric plate 310 is attached to a substrate 320. Aportion of the piezoelectric plate 310 forms a diaphragm 315 spanning acavity 340 in the substrate. The cavity 340 does not fully penetrate thesubstrate 320. Fingers, such as finger 336, of an IDT are disposed onthe diaphragm 315. The cavity 340 has been formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings 342 in the piezoelectric plate 310. Thelateral (i.e. left-right as shown in the figure) extent of the cavity340 is defined by a lateral etch-stop 350. While the lateral etch-stop350 is shown in cross-section in FIG. 3B, it should be understood thatthe lateral etch-stop 350 is a continuous closed band of etch-stopmaterial that surrounds the cavity 340 such that the lateral etch-stop350 also defines the size of the cavity 340 in the direction normal tothe plane of the drawing. The vertical (i.e. the dimension normal to thesurface of the piezoelectric plate 310) extent or depth of the cavity isdefined by a vertical etch stop 352. In this case, the cavity 340 has arectangular, or nearly rectangular, cross section.

The lateral etch-stop 350 and the vertical etch-stop 352 are formed fromthe same etch-stop material or different etch-stop materials, all ofwhich are substantially impervious to the process and etchant used toform the cavity 340. The lateral etch-stop 350 and the verticaletch-stop 352 may be materials that are not etched by the etch processused to form the cavity, or that are etched sufficiently slowly that theetch-stop constrains the lateral extent of the cavity 340 and thevertical etch-stop 352 defines the depth of the cavity. When thesubstrate 320 is silicon, the etchant may be, for example, XeF₂. Thelateral etch-stop 350 and the vertical etch-stop 352 may be, forexample, SiO₂, Si₃N₄, a metal oxide, a metal nitride, a glass, aceramic, or a polymer material. Different etch-stop materials anddifferent etchants may be used with different substrate materials.

Description of Methods

FIG. 4 is a simplified flow chart showing a process 400 for making anXBAR or a filter incorporating XBARs. The process 400 starts at 405 witha substrate and a plate of piezoelectric material and ends at 495 with acompleted XBAR or filter. As will be described subsequently, thepiezoelectric plate may be mounted on a sacrificial substrate or may bea portion of wafer of piezoelectric material. The flow chart of FIG. 4includes only major process steps. Various conventional process steps(e.g. surface preparation, cleaning, inspection, deposition,photolithography, baking, annealing, monitoring, testing, etc.) may beperformed before, between, after, and during the steps shown in FIG. 4.

The flow chart of FIG. 4 captures three variations of the process 400for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 410A, 410B, or 410C.Only one of these steps is performed in each of the three variations ofthe process 400.

The piezoelectric plate may be, for example, Z-cut, rotated Z-cut, orrotated Y-cut lithium niobate or lithium tantalate. The piezoelectricplate may be some other material and/or some other cut. The substratemay be silicon. The substrate may be some other material that allowsformation of deep cavities by etching or other processing.

In one variation of the process 400, one or more cavities are formed inthe substrate at 410A, before the piezoelectric plate is bonded to thesubstrate at 420. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. For example, thecavities may be formed using deep reactive ion etching (DRIE).Typically, the cavities formed at 410A will not penetrate through thesubstrate, and the resulting cavities will have a cross-section as shownin FIG. 3C (without the lateral or vertical etch-stops 350, 352).

At 420, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or other dielectric, may beformed or deposited on the mating surface of one or both of thepiezoelectric plate and the substrate. One or both mating surfaces maybe activated using, for example, a plasma process. The mating surfacesmay then be pressed together with considerable force to establishmolecular bonds between the piezoelectric plate and the substrate orintermediate material layers.

In a first variation of 420, the piezoelectric plate is initiallymounted on a sacrificial substrate. After the piezoelectric plate andthe substrate are bonded, the sacrificial substrate, and any interveninglayers, are removed to expose the surface of the piezoelectric plate(the surface that previously faced the sacrificial substrate). Thesacrificial substrate may be removed, for example, by material-dependentwet or dry etching or some other process.

In a second variation of 420 starts with a single-crystal piezoelectricwafer. Ions are implanted to a controlled depth beneath a surface of thepiezoelectric wafer (not shown in FIG. 4). The portion of the wafer fromthe surface to the depth of the ion implantation is (or will become) thethin piezoelectric plate and the balance of the wafer is effectively thesacrificial substrate. After the implanted surface of the piezoelectricwafer and device substrate are bonded, the piezoelectric wafer may besplit at the plane of the implanted ions (for example, using thermalshock), leaving a thin plate of piezoelectric material exposed andbonded to the substrate. The thickness of the thin plate piezoelectricmaterial is determined by the energy (and thus depth) of the implantedions. The process of ion implantation and subsequent separation of athin plate is commonly referred to as “ion slicing”. The exposed surfaceof the thin piezoelectric plate may be polished or planarized after thepiezoelectric wafer is split.

Conductor patterns and dielectric layers defining one or more XBARdevices are formed on the surface of the piezoelectric plate at 430.Typically, a filter device will have two or more conductor layers thatare sequentially deposited and patterned. The conductor layers mayinclude bonding pads, gold or solder bumps, or other means for makingconnection between the device and external circuitry. The conductorlayers may be, for example, aluminum, an aluminum alloy, copper, acopper alloy, molybdenum, tungsten, beryllium, gold, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layers and the piezoelectric plate.The conductor layers may include bonding pads, gold or solder bumps, orother means for making connection between the device and externalcircuitry.

Conductor patterns may be formed at 430 by depositing the conductorlayers over the surface of the piezoelectric plate and removing excessmetal by etching through patterned photoresist. Alternatively, theconductor patterns may be formed at 430 using a lift-off process.Photoresist may be deposited over the piezoelectric plate and patternedto define the conductor pattern. The conductor layer may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 440, one or more dielectric layers may be formed by depositing one ormore layers of dielectric material on the front side of thepiezoelectric plate. The dielectric layers may be, for example, silicondioxide, silicon nitride, or some other material. The dielectric layersmay be deposited using conventional techniques such as sputtering orchemical vapor deposition. The one or more dielectric layers mayinclude, for example, a dielectric layer selectively formed over theIDTs of shunt resonators to shift the resonance frequency of the shuntresonators relative to the resonance frequency of series resonators asdescribed in U.S. Pat. No. 10,491,192. The one or more dielectric layersmay include an encapsulation/passivation layer deposited over all or asubstantial portion of the device

In a second variation of the process 400, one or more cavities areformed in the back side of the substrate at 410B after all the conductorpatterns and dielectric layers are formed at 430. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back-side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In a third variation of the process 400, one or more cavities in theform of recesses in the substrate may be formed at 410C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device. The one or more cavities formed at 410C will notpenetrate through the substrate, and the resulting resonator deviceswill have a cross-section as shown in FIG. 3A.

In all variations of the process 400, the filter device is completed at460. Actions that may occur at 460 include depositing anencapsulation/passivation layer such as silicon oxide, silicon nitrideor silicon oxynitride over all or a portion of the device and/or formingbonding pads or solder bumps or other means for making connectionbetween the device and external circuitry if these steps were notperformed at 430. Other actions at 460 may include excising individualdevices from a wafer containing multiple devices; other packaging steps;and testing. Another action that may occur at 460 is to tune theresonant frequencies of the resonators within the device by adding orremoving metal or dielectric material from the front side of the device.After the filter device is completed, the process ends at 495.

Forming the cavities at 410A may require the fewest total process stepsbut has the disadvantage that the XBAR diaphragms will be unsupportedduring all of the subsequent process steps. This may lead to damage to,or unacceptable distortion of, the diaphragms during subsequentprocessing.

Forming the cavities using a back-side etch at 410B requires additionalhandling inherent in two-sided wafer processing. Forming the cavitiesfrom the back side also greatly complicates packaging the XBAR devicessince both the front side and the back side of the device must be sealedby the package.

Forming the cavities by etching from the front side at 410C does notrequire two-sided wafer processing and has the advantage that the XBARdiaphragms are supported during all of the preceding process steps.However, an etching process capable of forming the cavities throughopenings in the piezoelectric plate will necessarily be isotropic. Asillustrated in FIG. 3A, such an etching process will etch laterally(i.e. parallel to the surface of the substrate) as well as etchingnormal to the surface of the substrate. It may be difficult to controlthe lateral extent and shape of cavities etched in this manner.

FIG. 5A, FIG. 5B, and FIG. 5C (collectively “FIG. 5”) are a simplifiedflow chart of an improved process 500 for fabricating an XBAR with alateral etch-stop (for example, lateral etch-stop 350 in FIG. 3B). Tothe right of each action in the flow chart is a schematiccross-sectional view representing the end of each action. The process500 starts at 510 in FIG. 5A with a substrate 512 and a plate ofpiezoelectric material. The piezoelectric plate may be mounted on asacrificial substrate or may be a portion of wafer of piezoelectricmaterial as previously described. The process 500 ends at 595 in FIG. 5Cwith a completed resonator or filter device. The flow chart of FIG. 5includes only major process steps. Various conventional process steps(e.g. surface preparation, cleaning, inspection, deposition,photolithography, baking, annealing, monitoring, testing, etc.) may beperformed before, between, after, and during the steps shown in FIG. 5.

At 520, trenches 522 are formed in the substrate 512 in the locationswhere the lateral etch stop is desired. While the trenches 522 are onlyshown in cross-section in FIG. 5A, it must be understood that eachetch-stop trench is a ring completely around the perimeter of what willbecome a cavity. The trenches 522 may be formed by etching the substratethrough a suitable mask such as a photoresist mask or a hard mask. Thetrenches may be etched into the substrate using a suitable wet or dryetching process. For example, when the substrate is silicon, thetrenches may be formed using deep reactive ion etching (DRIE). Otheretching processes may be used on other substrate materials.

The depth d of the trenches 522 may be preferably, but not necessarily,greater than or equal to an intended maximum depth of the cavityadjacent to the etch-stop.

At 525, lateral etch-stops are formed by filling the trenches 522 withone or more etch-stop materials. The etch-stop material or materials maybe grown on the substrate and/or deposited onto the substrate usingconventional deposition processes such as thermal oxidation,evaporation, sputtering, or chemical vapor deposition. The etch-stopmaterial or materials may be any materials that will function toconstrain the lateral growth of the cavity to be etched in the substrate512. When the substrate 512 is silicon, suitable etch-stop materialsinclude silicon dioxide, silicon nitride, and aluminum oxide.

The lateral etch-stops formed at 520 may be a single material, as shownfor lateral etch-stop 524, which may one of the previously describedetch-stop materials deposited by a conventional process such assputtering or chemical vapor deposition. The lateral etch-stops may betwo or more materials as shown for lateral etch-stop 526. For example,when the substrate 512 is silicon, a layer of silicon dioxide may firstbe grown on the surface of the substrate 512 and the interior of thetrenches 522. Grown oxide typically has fewer pinholes and other defectsthan deposited materials. Subsequently, a second material may bedeposited over the grown oxide on the surface of the substrate 512 andwithin the trenches 522. As will be discussed subsequently, there may besome benefit to depositing a material other than silicon dioxide overthe grown oxide.

After the trenches 522 are filled with one or more materials to form thelateral etch-stops 524, 526, the surface of the substrate 512 will beuneven and must be planarized. Planarization may be performed bymechanical polishing, by chemo-mechanical polishing, or some othermethod.

At 530, a bonding layer 536 is formed on the planarized surface of thesubstrate. The bonding layer may silicon dioxide or some materialcapable of bonding to the piezoelectric material (typically lithiumniobate or lithium tantalate) to be used in the XBAR. The bonding layermay be formed by a conventional process such as evaporation, sputtering,chemical vapor deposition or molecular beam epitaxy.

Referring now to FIG. 5B, at 540, a piezoelectric plate 542 is bonded tothe bonding layer 536. Techniques for bonding the piezoelectric platewere previously described for action 420 in the process 400 of FIG. 4.The description of those techniques will not be repeated.

At 550, conductor patterns 552 are formed on the surface of thepiezoelectric plate 542. The conductor patterns include IDT fingers 554disposed on portions of the piezoelectric plate 542 where cavities willbe formed in the substrate. The structure of and techniques for formingthe conductor patterns were previously described for action 430 in theprocess 400 of FIG. 4. These descriptions will not be repeated.

At 555, one or more dielectric layers may be formed on the surface ofthe piezoelectric plate 542 over the conductor patterns 552. Thedielectric layers may include a layer 556 selectively formed over theIDT fingers of shunt resonators. The structure of and techniques forforming the dielectric layers were previously described for action 440in the process 400 of FIG. 4. These descriptions will not be repeated.

At 560, openings 562 are etched through the piezoelectric plate 542 andthe underlying bonding layer 536. The openings 562 may be circular holesor elongated slots or some other shape. As shown in FIG. 5B, theopenings 562 are adjacent to the area occupied by the IDT fingers 554.In other embodiments, there may be openings within the area of the IDTfingers. In all cases, the openings 562 are within regions encircled bythe lateral etch stops 524, 526.

Referring now to FIG. 5C, at 570, cavities 572 are etched into thesubstrate 512 using a first liquid or gaseous etchant introduced via theopenings 562. The lateral growth of the cavities 572 is constrained bythe lateral etch-stops 524, 526, and the shape of the bottoms of thecavities is uncontrolled. The depth of the cavities 572 is limited onlyby the duration of the etching process. Some lateral growth of thecavities may occur, as shown in FIG. 5C, if the depth of the cavitiesexceeds the height of the lateral etch-stops 524, 526. The shape of thecavities 572 shown in FIG. 5C is exemplary but not based on simulationor measurement of actual etched cavities.

Depending on the material and thickness of the bonding layer 536, it maybe necessary to remove the bonding layer material from the back side 582of the diaphragms. To this end, a second liquid or gaseous etchant isintroduced via the openings 562. If the lateral etch-stops 524, 526include the same material as the bonding layer 536, removal of thebonding layer material from the back side 582 of the diaphragms may alsoremove all or part of the lateral etch-stops.

The filter device is then completed at 590. Actions that may occur at590 include depositing an encapsulation/passivation layer such as SiO₂or Si₃O₄ over all or a portion of the device and/or forming bonding padsor solder bumps or other means for making connection between the deviceand external circuitry if these steps were not performed at 550. Otheractions at 590 may include excising individual devices from a wafercontaining multiple devices; other packaging steps; and testing. Anotheraction that may occur at 590 is to tune the resonant frequencies of theresonators within the device by adding or removing metal or dielectricmaterial from the front side of the device. After the filter device iscompleted, the process ends at 595.

FIG. 6A, FIG. 6B, and FIG. 6C (collectively “FIG. 6”) are a simplifiedflow chart of another process 600 for fabricating an XBAR. The process600 uses both a lateral etch-stop and a vertical etch-stop (for example,lateral etch-stop 350 and vertical etch-stop 352 in FIG. 3C). To theright of each action in the flow chart is a schematic cross-sectionalview representing the end of each action.

The process 600 starts at 610 in FIG. 6A with a substrate 612 and aplate of piezoelectric material. The substrate 612 is a laminatecomprising a device layer 614, a buried layer 616, and a handle layer618. For example, the device layer 614 may be silicon with a thicknessbetween 15 and 100 microns. The buried layer 616 may be a silicondioxide layer grown on one or both of the handle layer 618 and thedevice layer 614. The thickness of the buried oxide layer 616 may beabout 2 microns. The handle layer 618 may be a silicon wafer having athickness of about 250 microns to 1000 microns. The device, buried, andhandle layers are typically bonded together using a wafer bondingprocess. Other combinations of material may be used for the device,buried, and handle layers. The buried is an etch-stop material aspreviously described.

The piezoelectric plate may be mounted on a sacrificial substrate or maybe a portion of wafer of piezoelectric material as previously described.The process 600 ends at 695 in FIG. 6C with a completed resonator orfilter device. The flow chart of FIG. 6 includes only major processsteps. Various conventional process steps (e.g. surface preparation,cleaning, inspection, deposition, photolithography, baking, annealing,monitoring, testing, etc.) may be performed before, between, after, andduring the steps shown in FIG. 6.

At 620, trenches 622 are formed in the device layer 614 in the locationswhere the lateral etch stop is desired. While the trenches 622 are onlyshown in cross-section in FIG. 5A, it must be understood that eachetch-stop trench is a ring completely around the perimeter of what willbecome a cavity. The trenches 622 may be formed by etching the substratethrough a suitable mask such as a photoresist mask or a hard mask. Thetrenches may be etched into the substrate using a suitable wet or dryetching process. For example, when the substrate is silicon, thetrenches may be formed using deep reactive ion etching (DRIE). Otheretching processes may be used on other substrate materials.

So long as the process used to etch the trenches 522 does not etch theburied layer 616, the depth of the trenches 622 is determined by thethickness of the device layer 614.

At 625, lateral etch-stops are formed by filling the trenches 622 withone or more etch-stop materials. The etch-stop material or materials maybe grown on the substrate and/or deposited onto the substrate usingconventional deposition processes such as thermal oxidation,evaporation, sputtering, or chemical vapor deposition. The etch-stopmaterial or materials may be any materials that will function toconstrain the lateral growth of the cavity to be etched in the devicelayer 614. When the device layer 614 is silicon, suitable etch-stopmaterials include silicon dioxide, silicon nitride, and aluminum oxide.

The lateral etch-stops formed at 620 may be a single material, as shownfor lateral etch-stop 624, which may one of the previously describedetch-stop materials deposited by a conventional process such assputtering or chemical vapor deposition. The lateral etch-stops may betwo or more materials as shown for lateral etch-stop 626. For example,when the device layer 614 is silicon, a layer of silicon dioxide mayfirst be grown on the surface of the device layer 614 and the interiorof the trenches 622. Grown oxide typically has fewer pinholes and otherdefects than deposited materials. Subsequently, a second material may bedeposited over the grown oxide on the surface of the device layer 614and within the trenches 622. As will be discussed subsequently, theremay be some benefit to depositing a material other than silicon dioxideover the grown oxide.

After the trenches 622 are filled with one or more materials to form thelateral etch-stops 624, 626, the surface of the substrate 612 will beuneven and must be planarized. Planarization may be performed bymechanical polishing, by chemo-mechanical polishing, or some othermethod.

At 630, a bonding layer 636 is formed on the planarized surface of thedevice layer 614. The bonding layer may silicon dioxide or some materialcapable of bonding to the piezoelectric material (typically lithiumniobate or lithium tantalate) to be used in the XBAR. The bonding layermay be formed by a conventional process such as evaporation, sputtering,chemical vapor deposition or molecular beam epitaxy.

Referring now to FIG. 6B, at 640, a piezoelectric plate 642 is bonded tothe bonding layer 636. Techniques for bonding the piezoelectric platewere previously described for action 420 in the process 400 of FIG. 4.The description of those techniques will not be repeated.

At 650, conductor patterns 652 are formed on the surface of thepiezoelectric plate 642. The conductor patterns include IDT fingers 654disposed on portions of the piezoelectric plate 642 where cavities willbe formed in the substrate. The structure of and techniques for formingthe conductor patterns were previously described for action 430 in theprocess 400 of FIG. 4. These descriptions will not be repeated.

At 655, one or more dielectric layers may be formed on the surface ofthe piezoelectric plate 642 over the conductor patterns 652. Thedielectric layers may include a layer 656 selectively formed over theIDT fingers of shunt resonators. The structure of and techniques forforming the dielectric layers were previously described for action 440in the process 400 of FIG. 4. These descriptions will not be repeated.

At 660, openings 662 are etched through the piezoelectric plate 642 andthe underlying bonding layer 636. The openings 662 may be circular holesor elongated slots or some other shape. As shown in FIG. 6B, theopenings 662 are adjacent to the area occupied by the IDT fingers 654.In other embodiments, there may be openings within the area of the IDTfingers. In all cases, the openings 662 are within regions encircled bythe lateral etch stops 624, 626.

Referring now to FIG. 6C, at 670, cavities 672 are etched into thedevice layer 614 using a first liquid or gaseous etchant introduced viathe openings 662. The lateral growth of the cavities 672 is constrainedby the lateral etch-stops 624, 626. The depth of the cavities 672 islimited by the buried layer 616, which functions as the verticaletch-stop.

Depending on the material and thickness of the bonding layer 636, it maybe necessary to remove the bonding layer material from the back side 682of the diaphragms. To this end, a second liquid or gaseous etchant isintroduced via the openings 662. If the lateral etch-stops 624, 626include the same material as the bonding layer 636, removal of thebonding layer material from the back side 682 of the diaphragms may alsoremove all or part of the lateral etch-stops. Similarly, when thebonding layer is silicon dioxide, removal of the bonding layer materialfrom the back side 682 of the diaphragms may also remove all or part ofthe buried oxide layer 616 beneath the cavities.

The filter device is then completed at 690. Actions that may occur at690 include depositing an encapsulation/passivation layer such as SiO₂or Si₃O₄ over all or a portion of the device and/or forming bonding padsor solder bumps or other means for making connection between the deviceand external circuitry if these steps were not performed at 650. Otheractions at 690 may include excising individual devices from a wafercontaining multiple devices; other packaging steps; and testing. Anotheraction that may occur at 690 is to tune the resonant frequencies of theresonators within the device by adding or removing metal or dielectricmaterial from the front side of the device. After the filter device iscompleted, the process ends at 695.

FIG. 7A and FIG. 7B (collectively “FIG. 7”) are a simplified flow chartof another process 700 for fabricating an XBAR. The process 700 usesboth a lateral etch-stop and a vertical etch-stop (for example, lateraletch-stop 350 and vertical etch-stop 352 in FIG. 3C). To the right ofeach action in the flow chart is a schematic cross-sectional viewrepresenting the end of each action. The later steps of the process 700are similar to the steps 640-695 of the process 600 of FIG. 6. Thedescription of these steps will not be repeated. Various conventionalprocess steps (e.g. surface preparation, cleaning, inspection,deposition, photolithography, baking, annealing, monitoring, testing,etc.) may be performed before, between, after, and during the stepsshown in FIG. 7.

The process 700 starts at 710 in FIG. 7A with a substrate 712 and aplate of piezoelectric material. The substrate 712 may be a siliconwafer or a wafer of another material that allows the formation of deepcavities by etching or some other process. The piezoelectric plate maybe mounted on a sacrificial substrate or may be a portion of wafer ofpiezoelectric material as previously described.

At 720, recesses 722 are formed in the substrate 712 in the locationswhere the lateral etch stop is desired. While the recesses 722 are onlyshown in cross-section in FIG. 7A, it must be understood that eachrecess 722 is a three-dimensional created by removing material from thesubstrate. Each recess 722 may have a cross-sectional shape (normal tothe plane of the drawing) that is a rectangle, a regular or irregularpolygon, oval, or some other shape. The recesses 722 may be formed byetching the substrate through a suitable mask such as a photoresist maskor a hard mask. The recesses 722 may be etched into the substrate usinga suitable wet or dry etching process. For example, when the substrateis silicon, the recesses may be formed using deep reactive ion etching(DRIE). Other etching processes may be used on other substratematerials.

At 725, lateral and vertical etch-stops 732 are formed by coating thesubstrate 712, including the bottom and sides of the recesses 722 withone or more etch-stop materials. The etch-stop material or materials maybe grown on the substrate and/or deposited onto the substrate usingconventional deposition processes such as thermal oxidation, physicalvapor deposition, or chemical vapor deposition. The etch-stop materialor materials may be any materials that will function to constrain thelateral and vertical growth of the cavity to be formed in the substrate.When the substrate 712 is silicon, suitable etch-stop materials includegrown or deposited silicon dioxide, silicon nitride, and aluminum oxide.

At 730, the recesses 722 are filled with a sacrificial material 736. Insubsequent process steps, a piezoelectric plate will be bonded to thesubstrate and the sacrificial material will be removed to form cavitiesunder the piezoelectric plate. The sacrificial material 736 can be anymaterial that can be subsequently be removed by etching, dissolving, orsome other process. The etch stop material 722 can be any material thatis impervious to the process used to remove the sacrificial material.For example, when the substrate is silicon, the etch-stop material maybe silicon dioxide and the sacrificial material may be polysilicon.Other combinations of substrate material, etch-stop material, andsacrificial material may be used.

After the recesses 722 are filled with sacrificial material 736, thesurface of the substrate 712 will be uneven and must be planarized.Planarization may be performed by mechanical polishing, bychemo-mechanical polishing, or some other method.

At 730 in FIG. 7B, a bonding layer 736 is formed on the planarizedsurface of the substrate 712. The bonding layer may silicon dioxide orsome material capable of bonding to the piezoelectric material(typically lithium niobate or lithium tantalate) to be used in the XBAR.The bonding layer may be formed by a conventional process such asphysical vapor deposition, chemical vapor deposition, or molecular beamepitaxy.

The process 700 then continues at 640 in FIG. 6B. The subsequent stepsof the process are as previously described with the exception that, at670, the cavities are formed by removing the sacrificial material fromthe recesses in the substrate.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

1. An acoustic resonator device comprising: a substrate having a frontsurface and a cavity and comprising a substrate material; a lateraletch-stop that defines a perimeter of the cavity, the lateral etch-stopcomprising a lateral etch-stop material different from the substratematerial; a single-crystal piezoelectric plate having opposing front andback surfaces, the back surface attached to the front surface of thesubstrate except for a portion of the piezoelectric plate forming adiaphragm that spans the cavity; and an interdigital transducer (IDT)formed on the front surface of the piezoelectric plate such thatinterleaved fingers of the IDT are disposed on the diaphragm.
 2. Thedevice of claim 1, wherein the piezoelectric plate and the IDT areconfigured such that a radio frequency signal applied to the IDT excitesa primary shear acoustic mode in the diaphragm.
 3. The device of claim2, wherein the piezoelectric plate is one of lithium niobate and lithiumtantalate.
 4. The device of claim 1, further comprising: a bonding layerdisposed between and in contact with the back surface of thepiezoelectric plate and the front surface of the substrate.
 5. Thedevice of claim 1, where in the substrate is a laminate comprising: adevice layer having a first surface and a second surface; a buriedvertical etch-stop layer having a third surface and a fourth surface,the third surface bonded to the second surface; and a handle layerhaving a fifth surface and a sixth surface, the fifth surface bonded tothe fourth surface, wherein the back surface of the piezoelectric plateis attached to the first surface, and the cavity extends from the backsurface of the piezoelectric plate though the upper device layer to theburied vertical etch-stop layer.
 6. The device of claim 5, wherein thedevice layer is silicon, the buried vertical etch-stop layer is silicondioxide, and the handle layer is silicon.
 7. The device of claim 1,wherein the lateral etch-stop material comprises one of silicon oxide,silicon nitride, silicon oxynitride, a metal oxide, a metal nitride aglass, a ceramic, and a polymer material.
 8. The device of claim 1,wherein the cavity is formed by an etch process; and the lateraletch-stop is substantially impervious to the etch process.
 9. The deviceof claim 1, further comprising: a vertical etch-stop that defines adepth of the cavity, the vertical etch-stop comprising a verticaletch-stop material different from the substrate material.
 10. The deviceof claim 9, wherein the vertical etch-stop material is one of siliconoxide, silicon nitride, silicon oxynitride, a metal oxide, a metalnitride a glass, a ceramic, and a polymer material.
 11. The device ofclaim 9, wherein the cavity is formed by an etch process; and thevertical etch-stop is substantially impervious to the etch process.12-24. (canceled)
 25. An acoustic filter device comprising: a substratehaving a front surface and a plurality of cavities and comprising asubstrate material; a plurality of lateral etch-stops, each lateraletch-stop defining a perimeter of a respective cavity from the pluralityof cavities, each lateral etch-stop comprising a lateral etch-stopmaterial different from the substrate material; a single-crystalpiezoelectric plate having opposed front and back surfaces, the backsurface attached to the front surface of the substrate except forportions of the piezoelectric plate forming a plurality of diaphragms,each diaphragm spanning a respective cavity from the plurality ofcavities; and a plurality of interdigital transducers (IDTs) of aplurality of resonators formed on the front surface of the piezoelectricplate such that interleaved fingers of each IDT are disposed on arespective diaphragm from the plurality of diaphragms.
 26. The device ofclaim 25, wherein the piezoelectric plate and all of the plurality ofIDTs are configured such that radio frequency signals applied to theplurality of IDTs excite primary shear acoustic modes in the respectivediaphragms.
 27. The device of claim 26, wherein the piezoelectric plateis one of lithium niobate and lithium tantalate.
 28. The device of claim25, further comprising: a bonding layer disposed between and in contactwith the back surface of the piezoelectric plate and the front surfaceof the substrate.
 29. The device of claim 25, where in the substrate isa laminate comprising: a device layer having a first surface and asecond surface; a buried vertical etch-stop layer having a third surfaceand a fourth surface, the third surface bonded to the second surface;and a handle layer having a fifth surface and a sixth surface, the fifthsurface bonded to the fourth surface, wherein the back surface of thepiezoelectric plate is attached to the first surface, and each cavityfrom the plurality of cavities extends from the back surface of thepiezoelectric plate though the device layer to the buried verticaletch-stop layer.
 30. The device of claim 29, wherein the device layer issilicon, the buried vertical etch-stop layer is silicon dioxide, and thehandle layer is silicon.
 31. The device of claim 25, wherein the lateraletch-stop material comprises one of silicon oxide, silicon nitride,silicon oxynitride, a metal oxide, a metal nitride a glass, a ceramic,and a polymer material.
 32. The device of claim 25, wherein theplurality of cavities are formed by an etch process; and the lateraletch-stop is substantially impervious to the etch process.
 34. Thedevice of claim 25, further comprising: a vertical etch-stop thatdefines a depth of the plurality of cavities, the vertical etch-stopcomprising a vertical etch-stop material different from the substratematerial;
 35. The device of claim 34, wherein the vertical etch-stopmaterial is one of silicon oxide, silicon nitride, silicon oxynitride, ametal oxide, a metal nitride a glass, a ceramic, and a polymer material.36. The device of claim 34, wherein the cavity is formed by an etchprocess; and the vertical etch-stop is substantially impervious to theetch process.
 37. The device of claim 25, wherein the plurality ofresonators includes one or more shunt resonators and one or more seriesresonators, the device further comprising: a dielectric layerselectively formed over the IDTs of shunt resonators to shift aresonance frequency of the shunt resonators relative to a resonancefrequency of series resonators.
 38. The device of claim 25, furthercomprising: an encapsulation/passivation layer deposited over all or asubstantial portion of the device.